The invention relates to a method for preparing polyaniline, in which aniline is polymerized in a reactor in a polymerization medium containing aniline and oxidant. The invention also relates to an apparatus for preparing polyaniline, comprising a polymerization reactor as well as feeding means for supplying aniline, polymerization medium and oxidant into the reactor.
Electroconductive polymers have been subject to extensive research in recent years. Polymers which show electrical conductivity due to the structure of the polymeric chain (intrinsically electroconductive polymers) provide a chance to replace metal conductors and semiconductor materials in many applications. One significant application is antistatic shielding (ESD) and electromagnetic shielding (EMI).
In the group of intrinsically electroconductive polymers, one technically promising polymer is polyaniline. Polyaniline is prepared by polymerizing an aniline monomer. The nitrogen atoms of monomer units are bonded to the para-carbon in the benzene ring of the next monomer unit. Polyaniline exists in two basic types: non-electroconductive base forms, of which the most common is emeraldine, and electroconductive salt forms which are obtained by doping the polymer with a suitable protonic acid. The polymerization method, which is the subject of this invention, can be used to produce both basic types, depending on further processing.
Polyaniline can be used in many applications which require electro-conductivity. A polyaniline which is known under the trademark PANIPOL and which is melt-processable and soluble and whose acidity is adjustable, can be used in several applications requiring electroconductivity. This polyaniline can be blended e.g. with a suitable matrix polymer by conventional blending methods used in the plastics industry, resulting in a homogeneous plastic material which maintains well its electroconductivity. Thus, the PANIPOL polyaniline is suitable for use in a variety of forms, such as
film materials and coatings for use in packaging materials of packaging industry, components or parts of components for electronic devices, and parts for different structures;
injection molded pieces for use in packages in electronics industry, containers suitable for the manipulation and transport of substances, casings such as casings for diskettes, components of copying machines, and pieces intended for industrial painting, shielding covers for sensitive electronic devices;
extrusion molding products for use in pipes, pipe parts and seals for products in the form of profiles; table boards, containers and container intermediate layers for products in the form of plates and laminates; and telecommunication cables, heat cables and inner and outer layers of power cables for products in the form of cables;
compression molding and rotational casting products for use in single pieces produced by compression molding and closed containers produced by rotational casting;
calendering products for use in floor and wall coverings, such as plastic carpets and wallpapers;
hot pressing and thermoforming products for use in conveyors, disposable gloves, elastoferic pieces and laminates,
elastomers for use in conveyors, roller belts, elastomeric components for copying machines, seals and rubber-like materials;
fibers for use in decorative materials (carpets, seat coverings and other textiles) in public rooms, vehicles, in explosion risk zones and in sterile rooms, clothing used under special conditions (headgear, footwear, work clothing), filters, dust collectors, membranes, conveyors.
In addition to the above-mentioned uses, polyaniline can be used in paints, varnishes and pigments, in corrosion protection and in parts for thermoelectric elements, accumulators or batteries.
The uses are particularly such in which antistatic shielding or EMI shielding is required, or in which electroconductive properties are otherwise useful. However, the list above is not meant to be exhaustive, and the use of polyaniline prepared by the method of the invention, or a form derived from it by further processing, is not necessarily limited to the alternatives described above.
Methods for polymerizing aniline, methods for further processing of polyaniline, and polymer properties are presented e.g. in European application publication 605 877, to which corresponds U.S. Pat. No. 5,436,317, European application publication 627 746 and U.S. Pat. No. 5,928,565. It is typical of all the known preparing methods to perform the polymerization as a batch process in a stirred tank reactor. The reaction is made by feeding a suitable oxidant, such as ammonium persulfate, into an aqueous solution of a protonic acid containing dissolved aniline in a reactor, to start polymerization. For example, according to U.S. Pat. No. 5,436,317, aniline and oxidant can be gradually fed into the aqueous solution of HCl, and the molar mass of the polymer can be influenced by the ratio of the feeding rates of aniline and the oxidant. The resulting doped polymer can be dedoped with a base to produce a non-electroconductive polyaniline product which can be re-doped with a suitable acid to produce an electroconductive polymer with the desired properties. By the selection of the re-doping acid, it is possible to influence the electro-conductive properties of the polymer.
By conventional methods, it is possible to prepare a large variety of polyaniline polymers whose properties can be adjusted according to the use. Up to date, the problem has been to control the reaction conditions to produce a polymer with a controlled uniform quality, for example a certain molecular weight distribution and/or particle size distribution.
It is an aim of the invention to present an improved preparing method whereby it is possible to better regulate the properties of polyaniline, either the non-electroconductive base form or the electroconductive form doped with an acid. To achieve this aim, the method according to the invention is primarily characterized in that the polymerization is performed in a reactor in which the ratio of the heat transfer surface to the reaction volume is at least 10/m, preferably at least 20/m. In practice, these conditions suitable for the final result are achieved e.g. in a tubular reactor by allowing a medium containing aniline and an oxidant to flow in a mixed flow through the tubular reactor. The reaction takes place preferably in a tubular reactor equipped with static mixers, wherein effective mixing is achieved. Static mixers are described in Perry""s Handbook (Perry, R. H., Green, D. W., eds., Perry""s Chemical Engineers"" Handbook, 7th ed., McGrew-Hill, New York 1998, pp. 18-32 to 18-33). The disclosure of the relevant pages of the Perry""s Handbook is incorporated herein by reference. The static mixers consist of stationary diverters inside the tubular reactor which force the fluid media to mix themselves through a progression of divisions and recombinations.
By the combined effect of the high ratio between the cooling surface area and the reaction volume, and the effective mixing, a polymerization temperature is achieved which is as stable as possible and controllable. The effective mixing means that the mixing time of the reactants is short in comparison with the reaction rate. The ratio between the heat transfer surface area and the reaction volume is obtained by dividing the surface area by the volume of the reaction medium, and its unit is m2/m3, i.e. 1/m.
In this invention, it has been found that the temperature control and the effective mixing are very important for the degree of uniformity of the different properties of the obtained polymer particles, i.e. they exhibit narrow distributions.
These conditions can be best achieved in a well-mixed tubular reactor in which differences in concentration are small in the area of the cross-section of the tube and there is hardly any dispersion in the axial direction. A high ratio between the heat transfer surface and the volume is obtained by a suitably small tube diameter. A high capacity can be obtained with several parallel tubes. It is also possible to achieve a high cooling surface to reaction volume ratio by installing the cooling pipe system inside the reactor tube itself. In this case, the cooling pipe system can be designed to be similar to a static mixer, where effective mixing is achieved.
As polymerization takes place in a mixed medium flowing through the tubular reactor, a homogeneous product is achieved, particularly with respect to particle size distribution and molecular weight distribution, or the distribution widths can be adjusted. The medium is effectively mixed and the conditions are uniform in the area of each cross-section perpendicular to the flowing direction. Furthermore, the polymer structure can be easily affected by changing the reaction conditions. By adjusting the temperature, particularly the length of the polymer chains can be regulated.
The method in which the polymerization takes place primarily in a tubular reactor, can be either a batch process or a continuous process. In the first mentioned case, the polymerization medium containing the reactant is circulated through the tubular reactor; in the latter case, the starting agents are supplied in a continuous flow to the tubular reactor, and the medium in which polymerization has proceeded to a certain point is taken out at the other end of the tubular reactor as a continuous flow to further processing steps.
According to yet another embodiment, at least the oxidant is fed as a separate flow to the tubular reactor. The tubular reactor makes it possible to feed the oxidant to the flow of the polymerization medium at separate consecutive points. Similarly, other starting agents can be fed more at a suitable point or at successive points, such as a dopant or a monomer or another necessary additive. Further, if the oxidation requires a catalyst, it, too, can be fed at successive points. The solution also makes it possible to feed possible polymerization termination agents at desired points. All in all, it is possible to prepare polyaniline qualities with various properties in an adjustable and controllable way. To increase the capacity, the medium can be led simultaneously through two or more tubular reactors coupled in parallel.
The apparatus according to the invention is, in turn, characterized in that the reactor is a tubular reactor to whose initial end is connected one or more inlets for feeding a polymerization medium and reactants and whose terminal end is provided with an outlet for discharging the polymerization medium and the polymer. Using a tubular reactor, the conditions can be made uniform, and the concentrations and the temperature of the reactants can be controlled at each point in the reactor. The tubular reactor can be a part of a batch reactor, wherein there is circulation between the outlet and the inlet via an intermediate container, preferably equipped with mixing, or the tubular reactor can be a part of a continuous process, wherein the outlet is connected to a line leading to further processing of the polymer. According to an advantageous embodiment, the tubular reactor is equipped with static mixers, wherein the agents are subjected to effective mixing as they flow through the tubular reactor. According to yet another advantageous embodiment, there are two or more tubular reactors coupled in parallel, wherein the capacity can be increased.
The reactor can be cooled by circulating a cooling liquid in a cooling jacket surrounding the reactor. Another alternative is to place a cooling pipe system inside the reactor tube, which cooling pipe system can be designed in the same way as a static mixer.